Backside-illuminated (BSI) imaging sensors include pixel arrays that are fabricated on the front side of a semiconductor substrate but can nonetheless capture images using light received through the backside of the substrate. During manufacture, the backside of silicon BSI sensors must be thinned by removing material from the backside of the substrate to allow nearby collection photodiodes to generate and collect the related charge. To reduce the color cross talk and improve quantum efficiency (QE) of the pixels in the pixel array, the substrate thickness is often reduced to a few microns.
After substrate thinning, a cleaning step is used to remove particles and other contaminants from the backside. Certain processes are currently used after substrate thinning to improve the sensor performance, such as backside dopant implantation followed by a laser/thermal annealing that is applied to activate the implanted dopants. These processes create several difficulties. Among other things, (1) the implanted dopant can go too deep and harm the short wavelength quantum efficiency (QE) of the pixels, (2) it is difficult to activate all the backside dopant and avoid un-activated defects, and (3) the use of high energy laser annealing can result in defects such as melting the substrate surface. These problems associated with current fabrication processes can cause undesirable problems in the resulting image sensors, such as high dark current and high white pixel count.